If I make customizations, then we add new ACT! users, will I need to customize individually each new computer or is there a way to basically "copy" the changes to additional users?

Here's how you share layouts etc:

At the source computer where you want to copy those customizations ( also called ‘ preferences’ ) from:

1) Close ACT!
2) Go to START -> MY DOCUMENTS
3) Copy the folder named DURKIN COMPUTING onto a shared drive

At the target computer
1) Close ACT!
2) Open the shared drive and highlight the DURKIN COMPUTING folder
3) Click CTRL+C on your keyboard
4) Go to START ->MY DOCUMENTS
5) Click CTRL+V on your keyboard and say [YES] to over-write the previous DURKIN COMPUTING folder

You can also do this for individual Durkin Products.
So to transfer the preferences for Task List Plus, copy that Task List Plus folder only from the source computer to the target computer.